Alloyed tungsten is widely used as a refractory material and as a heat-resistant construction material, for example in the production of heavy alloys, filaments for incandescent lamps, and in electronics applications. Tungsten carbides are used in machine building, for the production of tools and heavy-loaded wear parts, in the form of a powder metallurgy composite consisting of tungsten monocarbide with cobalt or another metal binder. These materials have enhanced wears and erosion resistance and significantly prolong the life of tools and parts operating under harsh conditions.
Alloying of metals is a complex physical-chemical phenomenon of significant practical interest. For example, alloying of iron with various amounts of carbon under various conditions might change its mechanical and physical properties dramatically from mild iron, to low carbon steel, high carbon steel and pig iron. The properties of steel, first of all its hardness, significantly depend on the carbon content and the form in which carbon is present in steel (e.g. as free cementite Fe3C or alternatively as an interstitial solid solution of carbon in iron).
Alloying should be distinguished from inclusions or simple mechanical mixing of several materials. For example free carbon inclusions into iron can have a negative effect on its mechanical properties, whereas as alloying can improve its mechanical properties.
In several practical applications tungsten is alloyed by substitution impurities and chemical compounds to achieve specific properties. For example JP 2003129164 describes a heavy tungsten-based alloy with 0.1-3 wt % of nickel and copper which has a specific weight 18.5-19.2 g/cm3. RU 2206629 describes a powder-metallurgy alloy of tungsten with 0.6-0.8 wt % Co; 0.2-0.4 wt % Ta; 0.2-0.4 wt % Ni; 2.0-5.0 wt % Fe; 0.1-0.2 wt % La, which is recommended as a heavy alloy with specific weight 17.8-18.2 g/cm3 and which can be deformed up to 50-80%.
To increase the re-crystallisation temperature and vibration resistance of an incandescent filament, powder metallurgy tungsten wire is alloyed with aluminium, potassium, silicon and rhenium 0.05-1.19 wt % [US 2003132707], or lanthanum oxide 0.05-1.00 wt % [U.S. Pat. No. 5,742,891], aluminium, potassium, silicon and rhenium 0.2-0.4 wt % [U.S. Pat. No. 6,624,577], or to produce a duplex wire with a core of thorium oxide alloyed tungsten (W+ThO2) and an external shell made of rhenium [U.S. Pat. No. 5,041,041].
Modern electrode materials used for arc welding require a combination of arc stability and arc-resistance, that according to U.S. Pat. No. 5,512,240 is achieved by alloying tungsten with lanthanum boride 0.02-1.0 wt %. U.S. Pat. No. 5,028,756 recommends, for electrode wire fox electro-erosion cutting, tungsten with Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or with their oxides.
Tungsten alloys hold an important position among refractory construction materials. According to U.S. Pat. No. 5,372,661, an alloy consisting of tungsten, molybdenum and rhenium with up to 50 ppm of carbon added as a reducing agent has excellent erosion resistance, ductility, toughness and high re-crystallisation temperature. Transitional metals such as vanadium, chromium, manganese, iron, cobalt, nickel and rare earth metals are added to refractory materials, in particular to tungsten, to purify it from harmful additions of oxygen, nitrogen, carbon and hydrogen (U.S. Pat. No. 5,722,036).
This purification significantly improves the physical-chemical properties (corrosion-resistance, resistance to high temperatures, electrical conductivity) and technological properties (ductility, suitability for rolling or cutting).
In addition to the widely known hard metals based on tungsten monocarbide, U.S. Pat. No. 4,053,306 describes a composite material comprising tungsten carbide-steel, which has metallurgically-produced tungsten carbide particles 1-20 microns in size evenly distributed in the volume of carbon steel ox steel alloyed with nickel and cobalt.
In accordance with the aforementioned references, metallurgical tungsten is often alloyed for the purpose of purifying it from damaging admixtures and impurities. The admixtures and additives have a very low solubility in the tungsten matrix, and according to [Savitsky E. M., Burkhanov G. S. Metallurgy of Refractory and Rare Metals, (in Russian), Moscow, Nauka, 1971, p. 356] such solutions are possible with concentrations of up to 0.0001 wt %. Admixtures of higher concentration can strongly affect the mechanical properties of tungsten, for example excessive carbon could be most damaging by segregation on the grain boundaries, causing tungsten embrittlement.
All the aforementioned examples of alloying are related to metallurgical alloying, which involves either powder metallurgy or melting processes performed at high temperatures above 1200° C. Lakhokin Y. V., Krasovsky A. I.; Tungsten-Rhenium Coatings, (in Russian), Moscow, Nauka, 1989, p. 159 describes alloys of tungsten with rhenium produced by low-temperature CVD from the gas phase in a broad range of concentrations. As shown in this publication, the gas-phase alloying of tungsten with rhenium (up to 9 wt %) provides significant improvement of the mechanical properties of tungsten, simultaneously enhancing its strength and ductility.
Usually, tungsten produced from a mixture of tungsten hexafluoride and hydrogen contains up to 0.0015 wt % of hydrogen, 0.0042 wt % of oxygen, 0.0085 wt % of nitrogen, 0.0086 wt % of carbon and 0.012 wt % of metal impurities, and has a micro-hardness at the level of 490-520 kg/mm2 (Khusainov M. A. Thermal strength of refractory materials produced by Chemical Vapour Deposition, Leningrad, Leningrad University Publishing, 1979, p. 160). By way of rectification of the tungsten hexafluoride, the content of the impurities in the tungsten can be reduced down to: 0.0012 wt % of oxygen, 0.0016 wt % of nitrogen, 0.0010 wt % of carbon and 0.0053 wt % of metal impurities, while the micro hardness can be reduced to 450-490 kg/mm2. The authors of the above references tried to reduce the content of impurities to obtain plastic tungsten both by powder metallurgy and by chemical vapour deposition methods.
U.S. Pat. No. 4,427,445 describes a hard, fine-grained, internally stressed material of tungsten and carbon or tungsten, carbon and oxygen, which is produced by thermochemical deposition. The material consists primarily of a two phase mixture of pure tungsten and an A15 structure, the tungsten phase comprising between about 20% and 90% of the material, which has a hardness of greater than 1200 VHN, and an average grain size of less than 0.1 μm. A coating of this material is formed on graphite bars by heating these in a furnace and passing a gaseous mixture of tungsten hexafluoride, hydrogen and carbon- and oxygen-containing organic reagents (alcohols, ethers, ketones) over the bars. The 10-80% content of the A15 phase is easily detected by X-ray diffraction (XRD) analysis, the XRD spectrum containing lines both for metallic tungsten and the A15 structure. A typical XRD spectrum of such a structure is shown on FIG. 2 of the present application, in which the A15 structure is W3C. The coating material has high compressive stresses (above 1000 MPa) and microhardness up to 2500 VHN.
To reduce hardness and relax internal stresses, U.S. Pat. No. 4,427,445 recommends additional beat treatment at temperatures between 600 and 700° C. so as at least partially to decompose or transform the A15 phase, thereby reducing hardness of the material to at least half of its pre-heat treated hardness. However, due to the differences in thermal expansion coefficients of the coating and the substrate material, the heat treatment applies additional stresses. At temperatures above 600° C. many materials lose their mechanical properties and the shapes and dimensions of work pieces can be distorted.
Various comparative tests described in EP 1 158 070 A1 (discussed below) show that a coating consisting of a mixture of metal tungsten and W3C shows worse wear-resistance as compared to other tungsten carbides WC and W2C, as well as to a mixture of metal tungsten and W2C. The use of oxygen-containing organic reagents results in the formation of tungsten oxyfluorides which are difficult to reduce with hydrogen. The presence of tungsten oxyfluorides results in a deterioration of the mechanical properties of the coatings.
EP 1158 070 A1 describes new compositions based on tungsten carbide alloyed with fluorine in a process of crystallisation from the gas phase. It is shown that the gas-phase chemical reactions between tungsten hexafluoride, propane and hydrogen at a substrate temperature of 400-900° C. and pressure 2-150 kPa crystallise a layer of tungsten carbide with a thickness of 0.5-300 microns. The carbon containing gas is activated by heating to 500-850° C. before its introduction into the reactor. The ratio between the carbon-containing gas and hydrogen is varied from 0.2 to 1.7 and the ratio between tungsten hexafluoride and hydrogen is varied from 0.02 up to 0.12. By variation of these process parameters, it is possible to obtain the following tungsten carbide compositions: WC+C, WC, WC+W2C, W2C, W2C+W3C, W2C+W12C, W2C+W3C+W12C, W3C, W3C+W12C, W12C, WC+W, W2C+W, W3C+W, W12C+W, W3C+W12C+W.
This method allows production of all single-phase tungsten carbides, their mixtures, as well as mixtures with carbon and with metal tungsten. It should be emphasised that this disclosure relates to compositions based on tungsten carbide as the main phase, with tungsten being an admixture impurity. This has been proved by X-ray diffraction analysis.
These tungsten carbide compositions have high hardness, up to 3500 kg/mm2. The content of carbon in these coating materials can be up to 15 wt % and the content of fluorine up to 0.5 wt %. However, these materials are quite brittle like most other carbides, and cannot resist intensive load or an impact.
To reduce the brittleness of the carbide deposits, EP 1 158 070 proposes use of multi-layer deposits, consisting of alternating layers of tungsten and any of the tungsten carbides described above or their mixtures. The ratio of the thicknesses of the individual alternating layers can range from 1:1 up to 1:5.
Modern machine building requires materials which can resist harsh abrasive, erosive and corrosive environments and which have enhanced durability. Cemented or sintered carbide, also called hardmetal, is one of the most widely used types of wear-resistant materials.
Cemented carbide is a composite material consisting of tungsten monocarbide with a cobalt, or sometimes nickel or another metal, binder with a relatively low melting temperature. The binder content and composition can vary, as can the grain size of the tungsten carbide. This material can be produced by powder metallurgy sintering or alternatively by spraying, for example using plasma or high velocity oxy-fuel. The term “cemented carbide” will be used further in this text to describe this type of material. The particles of tungsten carbide give hardness, while cobalt gives toughness; as a result, this material shows excellent wear- and erosion-resistance. At the same time, however, cemented carbide has several drawbacks, in particular it is brittle, especially in thin-walled items or on sharp corners, and is expensive in manufacturing and machining, especially for items of complex shape. The cobalt binder can be attacked by corrosion.